Surface Plasmon Enhanced Upconversion Luminescence of Yb 3+/ Tm3+ and Yb3+/ Er3+ Systems Doped in NaYF4 on Au Nanopillar Arrays

Au nanopillar arrays, specifically designed to couple with 980 nm excitation light, is shown to enhance the upconversion luminescence of NaYF4:Yb 3+/Tm3+ and Yb3+/Er3+ nanoparticles embedded in PMMA. Spectrally--resolved reflectivity of the Au nanopillars arrays confirmed the existence of the surface plasmon polariton at 980 nm, near the Yb3+ absorption and an additional localized surface plasmon resonance around 750 nm. Confocal imaging of upconversion shows enhancement in the upconversion luminescence of NaYF4:Yb 3+/Tm3+ and Yb3+/Er3+ nanoparticles on the Au nanopillar array. While the SPP near 980 nm enhanced both the NaYF4:Yb3+/Tm3+ and Yb 3+/Er3 systems, the LSPR only enhance the near--infrared emissions of Tm3+ at 780--800 nm. The combined effect results in a significantly higher enhancement of the near--infrared emission when compared to the visible upconversion lines of Tm3+, consistent with the interpretation of sequential surface plasmon assisted absorption and emission at two separate and disparate energies. Further evidences for the SPP and LSPR enhancement of the upconversion system are provided by the time--resolved spectroscopy.